【Research Background】

      Traditional fossil fuel consumption and serious environmental problems have led us to a new form of green and renewable energy. In this case, hydrogen is considered to be a promising solution to these problems, because of its rich reserves and cleanliness, researchers have made great efforts to find sustainable and effective ways to produce hydrogen. In recent years, electrocatalytic water decomposition technology has been unprecedentedly developed and is considered to be one of the most promising hydrogen production methods. In general, the best hydrogen evolution reaction electrocatalysts are usually made from platinum or other precious metals, but their high price and limited resources limit their practical market applications. In recent years, many alternatives to platinum-based electrocatalysts have been explored, such as transition metal sulfides, borides, phosphides, carbides, and nitrides. However, their electrocatalytic performance is still lower than the most advanced HER catalysts. Therefore, designing a simple and effective strategy for obtaining highly active and noble metal-free catalysts will play a vital role in the development of hydrogen energy.

[Introduction]

       Academician Xie Yi from the University of Science and Technology of China published in Nano Energy: ultra-thin F-based functional group MXene sheet to achieve efficient electrochemical hydrogen production. There are not many studies on the electrocatalytic activity and active sites of MXene. Here, a single layer of Ti2CTx filled with fluorine groups is proposed to obtain synergy in the number of reactivity and reaction sites. Theoretical calculations and electrochemical tests on this material show that the fluorine-containing monolayer Ti2CTx has better proton adsorption properties, and the lower charge transfer impedance leads to better reactivity and higher density activity. Site. This F-containing Ti2CTx nanosheet has an initial overpotential of 75 mV and a larger charge exchange density of 0.41 mA cm-2.

[Research Highlights]

      In this paper, the effects of fluorine groups, which are often neglected in the study, are analyzed from three aspects of conductivity, reactivity and active sites. The theoretical calculation and experiments prove that the fluorine group has excellent HER performance.

[Graphic introduction]

Fig.1 Flow chart and SEM and AFM characterization of Ti2CTx nanosheets

Fig. 2 Physical characterization a) XRD image of Ti2CTx, Ti2AlC and PDF card b) High resolution transmission of Ti2CTx c) XPS of fluorine element d) Map of MAPPING element

Figure 3 State density and temperature impedance tests.

       Ti2C is metallic in nature by the fact that Ti2CTx is continuous near the Fermi level and the resistance increases with increasing temperature.

Figure 4 Electrochemical impedance test a) LSV polarization curve b) tafel slope c-d) material electrochemical double layer capacitance method to calibrate the active surface area e) EIS impedance test f) cycle stability test

The LSV polarization curve shows that the initial potential of the fluorine-containing Ti2CTx single layer is 75 mV, and the overpotential at 170 mA cm-2 is 170 mV (relative to the Ti2AlC and the layered Ti2C).

     The Tafel slope of the fluorine-containing Ti2CTx is 100 mVdec-1 (Ti2AlC 183 mV dec-1 and layered Ti2C 138 mV dec-1). The electrochemical double layer capacitance test of c and d shows that the fluorine-containing Ti2CTx has a relatively high active area.

[Summary of this article]

   The excellent electrochemical catalytic hydrogen production performance of Ti2CTx can be explained from the following four angles:

The metal nature of the Ti2CTx nanosheet itself ensures rapid electron transport from the catalyst surface to the support electrode, thereby facilitating the electrocatalytic reaction.

Fluorine-based functional groups promote proton adsorption kinetics by reducing the adsorption of hydrogen by Gibbs free energy, which is beneficial to increase the reactivity of the active site and is beneficial to hydrogen production.

The fluoro group functional group can lower the charge transfer resistance and promote the movement of the electrode kinetics to hydrogen.

The ultra-thin thickness of Ti2CTx nanosheets provides a larger electrochemical surface area and a stronger catalytically active site than bulk nanosheets, which greatly contributes to its catalytic performance.

       This paper demonstrates that the surface-rich Ti-rich Ti2CTx nanosheet is an efficient and stable electrocatalyst for hydrogen evolution reaction with an initial overpotential of 75 mV and an exchange current density of 0.41 mA cm−2. The experimental observation combined with DFT calculations shows that the F-rich functional group not only promotes the adsorption kinetics of ions, but also reduces the charge transfer resistance, which gives Ti2CTx nanosheets excellent reaction reaction sites and mild electrode reaction kinetics. Electrocatalytic activity. At the same time, due to the synergistic effect between the ultrathin thickness of Ti2CTx nanosheets and metal behavior, it has a large number of active sites and high conductivity, which improves its activity. Thanks to these advantages, the fluorine-rich functional group Ti2CTx nanosheets finally achieved higher performance than the layered Ti2CTx and the original Ti2AlC. This study will provide valuable guidance for designing MXene-based materials with optimal electrocatalytic activity.

Literature link:

Https://doi.org/10.1016/j.nanoen.2018.03.022

Source: WeChat public number MXene FrontierFigure